1. Field of the Invention
This invention relates to a charge-transfer device using the principle of charge-coupled devices (hereinafter, abbreviated as CCDs), and more particularly to a charge-transfer device with an improved charge-sensing section.
2. Description of the Related Art
Recently, solid-state imaging systems using CCDs, a type of charge-transfer device, have been used in video cameras, electronic still cameras, and the like. The solid-state imaging system comprises a photoelectric conversion section for converting light into an electric signal (signal charge), a charge-transfer section for transferring the signal charge, and a charge-sensing section for sensing the transferred signal charge and converting it into a voltage signal. The charge-transfer device is made up of the charge-transfer section and the charge-sensing section.
One known charge-sensing section provided in the charge-transfer device is a floating diffusion amplifier (hereinafter, abbreviated as an FDA) placed next to the final stage of the charge-transfer section. The FDA contains a floating diffusion layer (hereinafter, abbreviated as an FD layer) provided between the output terminal of the charge-transfer section and a charge-releasing drain (reset drain). When being transferred from the charge-transfer section to the FD layer, the signal charge is temporarily stored in the FD layer, whose potential thus changes. The potential change at the FD layer is transmitted through an interconnection to the gate electrode of a driver transistor in a source follower circuit, causing the gate potential of the transistor to vary. The source follower circuit amplifies the change of the gate potential to produce a voltage signal, thereby sensing the signal charge. After the charge is sensed and the signal is outputted, the signal charges in the FD layer are released from the charge-releasing drain by opening the reset gate.
FIG. 14 is a schematic diagram of a conventional FDA charge-sensing section, particularly a source follower circuit.
At the surface of a semiconductor substrate 210, an FD layer 211 and a reset drain 212 are formed. Provided above the semiconductor substrate are a CCD transfer electrode 213, an output gate electrode 214, and a reset gate electrode 215, with a SiO.sub.2 film 230 underlying these electrodes. The FD layer 211 is electrically connected to the gate electrode of a transistor 221. The source of the transistor 221 is connected to the drain of a load transistor 222, thus forming a first-stage source follower circuit. The source of the transistor 221 is also connected to the gate electrode of a transistor 223. The source of the transistor 223 is connected to the drain of a load transistor 224, thus forming an output-stage source follower circuit. The gate electrodes of the load MOS transistors 222, 224 are both connected to the ground.
In the FDA charge-sensing section thus constructed, a parasitic capacity ascribed to the wiring from the FD layer to the gate electrode of the driver transistor in the source follower circuit is large. Because of this, the change of potential per stored charge cannot be made larger, and therefore it is difficult to sense charges with high sensitivity. It is also difficult to reduce kTC noise caused by release of the signal charge.
FIG. 15 shows the waveform of voltage V.sub.RS at the reset gate electrode 215 and that of the output voltage V.sub.OUT. As shown in the figure, the waveform of output voltage V.sub.OUT always contains noise. The noise is formed such that thermal noise generated at the reset gate electrode 215 is dominant over period T21 where the reset gate electrode 215 is on, and thermal noise created at the transistor 221 is dominant over period T22 where the reset gate electrode 215 is off and the signal charges are injected into the FD layer 211. Period T2b corresponds to a case where the signal charges are absent, and period T2w corresponds to a case where the signal charges are present.
In order to reduce noise in the FDA, the constant currents in the depletion-mode MOS transistors 222, 224 acting as loads in the source follower circuits may possibly be decreased. Decreasing the constant currents in the source followers, however, reduces the mutual conductance of the source followers, making it impossible for the FDA to respond rapidly. Therefore, the necessity of ensuring fast response makes it very difficult to decrease noise in the FDA. In addition, because the constant currents in the load MOS transistors 222, 224 cannot be reduced, there arises the problem of being unable to reduce the power consumption in the FDA.
To overcome these problems, a charge-transfer device with a charge-sensing section has been developed, as shown in U.S. Pat. No. 4,984,045 issued on Jan. 8, 1991. The charge-transfer device disclosed in the publication uses a buried channel layer of a charge-sensing MOS transistor placed next to the final stage of the charge-transfer section.
Because this structure has neither contact region nor interconnection for connecting the FD layer to the driver transistor of the source follower circuit, a parasitic capacity at the charge-sensing section can be decreased. This makes it possible to make the potential change per stored charge larger, thereby enabling charge to be sensed with high sensitivity. Because the stored charges can be discharged completely with the help of floating and control gates, it is possible to eliminate kTC noise generated at the reset gate.
with this structure, however, a higher power-supply voltage (e.g., -80 V) than a commonly used CCD power-supply voltage (e.g., 15 V) is required to operate the control gate, as described in Matsunaga, et al., "A Highly Sensitive On-Chip Charge Detector for CCD Area Image Sensor," IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 26, No. 4, April 1991. Furthermore, use of two layers of gates, the floating gate and the control gate, complicates the structure of the output sensing section and its manufacturing process.